This invention relates to a cathode, and particularly to structures and methods of operating the same in which a directly heated line cathode is employed.
Conventional kinescope tubes are generally limited to sizes and shapes which preclude realization of a flat image display device. One approach to obtain such a display device has been to employ a substantially flat tube having therein a line cathode. In this structure, the line cathode is utilized in combination with means for guiding electron beams therefrom to selectively strike predetermined portions of a cathodoluminescent screen. For examples of such structures see: copending applications of Stanley, Ser. No. 607,492, filed Aug. 25, 1975, entitled "FLAT ELECTRON BEAM ADDRESSED DEVICE, now U.S. Pat. No. 4,031,427, issued June 21, 1977"; and of Anderson et al., Ser. No. 615,353, filed Sept. 22, 1975, entitled, "GUIDED BEAM FLAT DISPLAY DEVICE, now U.S. Pat. No. 4,028,582, issued June 7, 1977".
One problem with the previously mentioned flat display device is the use of an indirectly heated cathode. The indirectly heated cathode is undesirable as its structure is generally more complex than a directly heated cathode. However, the use of a directly heated cathode has heretofore been discouraged because of other problems; the voltage drop across the line cathode due to the heating current usually exceeds the value allowable for injection into a series of guides operated at common voltages, and the voltage drop across the line cathode during operation often degrades the uniformity obtainable in a display device employing the same.
Thus, it would be desirable to develop a method and structure which employs a directly heated line cathode for use in a flat image display device.